Syringes are often used to pump heparin or other drugs to a blood flow line of a medical device at very low flow rates, e.g., 1/2 to 6 milliliters per hour, by driving the plunger with a stepper motor. As syringes of different dimensions may be used on the same machine model, it can be difficult to correlate the position of the plunger driving mechanism with the end of travel of the plunger, something desired to accurately determine when the syringe has been emptied and to shut off the stepper motor driving it. Some prior art heparin pumps employ clutches that slip when the plunger has reached the end of travel, to prevent damage to machine parts.